This invention relates to apparatus for picking up signal currents from a signal line, and more particularly to apparatus of the type wherein a thyristor is turned ON in response to a voltage induced in a pickup coil energized by signal current.
A signal pickup circuit of the type including an annular or closed magnetic core wound with a pickup coil, and a signal line or conductor linking the magnetic core and wherein a voltage pulse induced in the pickup coil when a signal current flows through the signal line is used to render ON a suitable detector such as a thyristor for producing a detection output, is widely used in such applications as a fire alarm system in which a plurality of sensors are arranged in a plurality of groups, and groups of alarming devices, smoke exhaust devices, fire extinguishing devices or operating devices for fire preventing doors are operated in accordance with the signals transmitted from a particular group of the sensors, or these safety devices in a particular block are controlled in accordance with the signal from a particular sensor, or these safety devices are controlled in accordance with the combinations of these control systems. A pickup circuit constructed as shown in FIG. 1 has been used for such applications. In FIG. 1, 7 designates a signal line through which a signal current, for example a signal current produced by a sensor of the type described above, is caused to flow. The signal line 7 links an annular or closed magnetic core 8 wound with a pickup coil 9. One terminal of the pickup coil 9 is connected to the gate electrode G of a thyristor or a silicon controlled rectifier SCR via a diode D and the other terminal is connected to the cathode electrode K of the thyristor. A resistor Rg is connected across the gate and cathode electrodes of the thyristor, while the anode electrode A and the cathode electrode K of the thyristor are connected in a closed circuit including a detection relay 10 and a source of direct current 14. The signal receiver 15 comprising source 14 and relay 10, and the signal transmitter 16 including core 8, pickup coil 9 and thyristor SCR are generally installed spaced apart and are interconnected by conductors 17 and 18. As shown, the receiver 15 is provided with a common input terminal 19 and a plurality of input terminals 19-1 . . . 19-n for respective sensors.
With the circuit construction shown in FIG. 1, when a signal current i flows through the signal line 7, a voltage proportional to the differentiated value of the signal current i is induced in the pickup coil 9 which is applied to the gate electrode G of the thyristor SCR thus triggering the same. Then a closed circuit is established through source 14, relay 10, terminal 19, conductor 17, thyristor SCR, conductor 18 and terminal 19-1 thereby energizing the relay 10. The closure or opening of the contact (not shown) of the relay detects the signal current i.
With such circuit construction, however, the magnitude of the voltage induced in the pickup coil 9 is greatly influenced by the magnitude of the input impedance of the load circuit connected to the pickup coil 9. More particularly, when a signal current i having a rectangular waveform as shown in FIG. 3a flows, although momentary, an extremely high voltage pluse would be generated where the load impedance is high, whereas when the load impedance is low, the voltage pulse would be relatively low as shown in FIG. 3b. Accordingly, with circuit construction shown in FIG. 1, as the input impedance across the gate and cathode electrodes G and K of the thyristor SCR is low it is impossible to produce a voltage pulse by the pickup coil 9 which is sufficiently high to trigger the thyristor. To obviate this difficulty, it has been necessary to use a so-called transformer element comprising the magnetic core 8 and the pickup coil 9 and having a large output. However, when a transformer element having large output is used, it will also respond to the noise current that may present on the signal line concurrently with the signal current thus causing misoperations and degrading the S/N ratio.
To solve this problem, although the thyristor SCR may be substituted by a high sensitivity programmable unijunction transistor (PUT) having a high input impedance, such a unijunction transistor is liable to be misoperated by the noise current from the source so that use thereof introduces a new trouble.